Current sensors are employed in numerous applications, such as light sensors, temperature sensors, gas sensors, magnetic sensors, motion sensors, proximity sensors, etc. A current sensing circuit typically comprises a suitable transducer that responds to an external stimulus, such as a photodiode responding to changing light, a magnetoresistive sensor responding to changing magnetic flux, or a tunneling current proximity probe responding to the proximity of elements, such as the proximity of a head to a disk in a disk drive. Typically the resistance of a current sensing transducer changes in response to the external stimulus so that when a bias voltage is applied to the transducer, the change in resistance caused by a change in the external stimulus results in a corresponding change in current passing through the transducer. Accordingly, a current sensor is typically employed to detect the change in current and thereby detect changes in the external stimulus.
Certain current sensing transducers, such as the aforementioned tunneling current proximity sensor, exhibit an extremely small change in resistance as the stimulus changes, and therefore they transduce an extremely small current (e.g., nanoamperes). There is, therefore, a need for an extremely sensitive current sensor capable of accurately detecting the extremely small currents generated by certain current sensing transducers.